The invention is based on a device for regulating the fuel-air ratio of the operating mixture being combusted in an internal engine using a .lambda. sensor, which is connected with a regulating device for influencing the fuel-air ratio, and having a device for monitoring the operational readiness of the .lambda. sensor. In order to ascertain the internal sensor resistance affecting the sensor readiness, a reference voltage is switched opposite to the sensor voltage by the interposition of a resistor. The resultant voltage (S.sub.r) at the .lambda. sensor output is capable of being monitored, by at least two comparator devices having different threshold values, as to the presence of a minimum stroke indicating the operational readiness of the .lambda. sensor. The outputs of the comparator devices are connected with a logic processing circuit whose output signal is evaluated by an evaluation circuit as a standard for the operational readiness. This evaluation circuit generates a first signal which indicates the operational readiness and/or establishes the operation of the regulating device. A second signal indicates the non-readiness for operation of the .lambda. sensor and/or shuts down the operation of the regulating device and switches the device over to open-loop control.
In a device of this kind (for example, U.S. Pat. No. 4,208,993), one of the comparator devices serves to ascertain whether the sensor signal is larger or smaller than an average voltage value, which, as the voltage value determining the regulating point, is located within the voltage jump of the lambda sensor output signal when .lambda.=1. The regulating device is controlled with the output value of this comparator device. In contrast, the output values of both comparator devices taken together represent an expression of the operational readiness of the lambda sensor. This arrangement has the disadvantage, however, that the threshold values for the comparator devices and the reference voltage are picked up directly from a single voltage divider; because the comparator devices draw an electric current, this affects the magnitude of the reference voltage and thus affects the regulating point as well. Furthermore, the adjustment range of the reference voltage is limited by the location of the threshold values, and the load resistance by way of which the reference voltage is applied to the sensor output and is fixed in its value. Because of the sensitivity of the reference voltage to the withdrawal of current from the voltage divider, there are limitations placed on the possible opportunities for evaluating the resultant voltage at the lambda sensor output by means of additional evaluation devices.